Dielectric Spectroscopy
Conventional impedance
Conventional impedance methods measure the frequency dependent voltage-current- characteristics of suspensions of particles (e.g. biological cells). To calculate the particle properties complex electric models for the impedance chamber need to be applied. The models depend on solution volume and electrode impedance, as well as for non-spherical objects, on particle orientation. Furthermore, the models cannot unambiguously assign distributions of properties to individual particles or to a distribution of properties within the suspension. With biological cells there are sometimes even problems in determining the correct volume concentration.
Dielectric Particle Spectroscopy (DPS)
Such drawbacks are overcome by dielectric particle spectroscopy. Different methods can be applied, all of them
measuring the field frequency dependence of individual particle movement induced by AC-fields. In any case particle
movement bases on the interaction of the external field with induced (or permanent) charges at the particle - medium
interface. Depending on the frequency dependent polarizability of the suspension medium relative to that of the
particle, medium or particle polarization dominates. Any dispersion process causes an amplitude and a phase shift
between the external field and the induced dipole moment. These processes are commonly investigated by two strongly
related techniques, dielectrophoresis (DP) and electrorotation (ER). These methods are independent of solution
volume and electrode impedance since the measured effect, particle movement, is caused by the difference in
polarizability of external medium and the particle itself. Extremely low particle concentrations can be used
thereby avoiding particle-particle interactions.
DP and ER
In DP, translational motion of particles in an inhomogeneous field towards (pos. DP) or away (neg. DP) from regions of high field is induced. Positive DP occurs when the particles' polarizability is better, negative DP when it is worse than that of the suspension medium. Dielectric dispersions mediate changes of the DP-force with frequency. Such dispersions are characteristic for Maxwell-Wagner, Debye or complex electrochemical charge transfer processes. In ER, a rotating field induces a dipole moment on the particles which rotates at the same angular frequency. In case of dispersion a spatial phase shift between the rotating external field and the induced dipole moment occurs. Depending on the relative polarizability, the interaction of dipole moment and external field then induces a torque causing individual particle rotation against (antifield rotation) or in (cofield rotation) the rotation direction of the field. The particle rotation speed depends on rotational friction and is typically in the order of one revolution per second.
Advancements
The basic idea of dielectric particle spectroscopy is to analyze the frequency dependence of DP and ER. Recently, significant progress was achieved:
- The conductivity limitation imposed by macroscopic measuring chambers was overcome by ultramicroelectrode chambers.
- Light scattering techniques like Electrorotational Light Scattering (ERLS) or Phase Analysis Light Scattering (PALS) were employed to overcome the drawbacks of microscopic observation of individual particles.
More information: Jan Gimsa